Composite materials are materials formed from a plurality of different constituent materials, in order to achieve preferable properties, such as increased strength and decreased weight. They are well known for use in structural components, particularly in situations where minimising weight is important, such as in automotive and motorsport industries. Many composite materials comprise fibre reinforcement in a polymeric matrix.
Cylindrical structures made of composite materials are commonly manufactured using filament winding techniques, in which filaments formed from a variety of materials (such as carbon, glass or aramid e.g. Kevlar fibres) are wound under tension around a mould at an angle appropriate to the application. A high winding angle (hoop winding) can provide resistance to internal or external pressure, whereas a low winding angle (helical or closed winding) can provide resistance to longitudinal bending and axial tension or compression. The wound filaments are embedded in a matrix, generally of polymer resin, which may be applied to the filaments before or after winding or embedded in the fibres (pre-impregnated). The resin matrix is then cured, typically being heat treated, in order to set the filaments in place. However, tubes formed by conventional filament winding techniques are highly permeable due to the nature of the material. Commonly used anhydride curing epoxy resins have a relatively high specific permeability making the resulting composites permeable to low molecular weight fluids. Furthermore, permeation rate increases as molecular weight of contained fluids decreases.
High pressure fluid storage is a growing sector in energy, automotive and aerospace markets with a focus on low molecular weight fluids such as hydrogen, nitrogen and helium of prime interest. Current pressure vessels used to store gases and fluids at high pressure are often made of steel and therefore are not weight optimised, reducing the practicality of their use in many situations. Composite materials are often used to reduce weight of pressure vessels or to allow elevated pressures compared to metallic alternatives. Due to the permeability of presently manufactured composites, composite pressure vessels must be rendered impermeable, for example by using metallic liners. However, use of discrete liners increases the weight of the vessel and can create a corrosion risk between dissimilar materials. It would therefore be desirable to remove the need for impermeable liners.